Polyurethane foam has been utilized for thermal insulation in buildings, storage tanks, freezers, freezer trucks and ships, railway cars, etc., for many years. To date, there have been basically three methods of applying the foam:
(a) A given thickness of the foam can be sprayed onto an existing surface. When cured, the foam can then be covered with a second surface, such a plywood, sheetrock, etc.
(b) The mixed chemicals can be poured into a cavity between two surfaces which are strong enough to withstand the post-expansion pressures. Often, the surfaces are weak surfaces, such as plywood or sheetrock. Such surfaces must be reinforced with forms, much like concrete forms, which are removed after the foam has cured.
(c) Slabs of foam can be pre-manufactured by pouring the mixed chemicals into suitable molds, usually 4 ft..times.4 ft..times.8 ft., and then sawing the cured polyurethane foam into slabs having a thickness of 1 inch up to 4 inches. The slabs can then be placed into attics, between walls, around storage tanks, etc.
Polyurethane foam is normally about 2 lbs. per cu. ft. density for insulation purposes. It is blown or foamed with water or Freon 11, which has a boiling point of 74.6.degree. F. Occassionally, foam is "frothed", or partially frothed, with Freon 12, which has a much lower boiling point, so that the foam is at least partially expanded as it comes from the mixing nozzle. Sometimes, the two components are run through heaters and through heated hoses, so as to cause more immediate expansion of the foam. By all these methods, however, the foam is never more than about 80 percent expanded when it emerges from the mixing nozzle. The subsequent post-expansion creates pressures which will blow apart walls in normal buildings.
U.S. Pat. No. 3,184,419 discloses a method for the production of polyurethane foam material. The method comprises incorporating a soluble inert gas under pressure in the liquified state into a liquid composition containing a polymer and a compound capable of reacting therewith to form a polyurethane foam material. The inert gas is introduced into the liquid composition at a temperature below that at which substantial reaction between the polymer and the reactant compound takes place. The liquid composition is foamed by reducing the pressure of the mixer and raising the temperature of the foam to convert the composition into an elastomer. Listed among suitable inert gases are carbon dioxide and halogenated methanes, including Freon 13 and Freon 23. While the patent discloses mixing the carbon dioxide or other inert gas into the prepolymer at sufficient pressure to maintain it as a liquid, the pressure of the mixture is allowed to drop to 300 p.s.i. in the mixing chamber. At least half of the expansion of the foam occurs in the mixing chamber at this pressure. When released from the mixing chamber to atmospheric pressure, there is simply not enough pressure drop to totally expand the remainder of the mixture. Furthermore, water is present in the system, which automatically means much post-expansion since water reacts with NCO groups to produce carbon dioxide. Moreover, the patented process requires cooling the inert gas to aid in maintaining it in the liquid state, which is a very expensive procedure.
U.S. Pat. No. 4,120,923 discloses a process for the manufacture of foamed polymeric materials. A blowing agent having a boiling point under normal pressure of less than 0.degree. C. is added to the resin in a pressure vessel under a pressure of 3 to 350 bars, at a temperature between 0.degree. C. and 50.degree. C. The mixture is subsequently expanded, in stages or continuously, and is cured with a curing agent. Examples of suitable blowing agents are air, carbon dioxide, helium, argon, nitrogen, volatile hydrocarbons, and volatile halogenated hydrocarbons, including Freon 13. The preferred blowing agent is carbon dioxide, since it demonstrates the greatest absorption in the case of most resins. This patent provides a very complicated process for the introduction of blowing agents. The gas is absorbed into the resin as the resin flows as a thin film over the disclosed apparatus. The patented process requires extremely complicated and expensive equipment which is totally inadequate for the rapid production of insulating foam. The patent does not disclose a process for the total pre-expansion of polyurethane foam.
U.S. Pat. No. 3,220,801 discloses apparatus for generating a polyurethane froth. The apparatus provides a mixing chamber into which are separately injected a polyisocyanate, an activator, and a volatile liquid, such as difluorodichloromethane, under pressure. The components are thoroughly mixed until the volatile liquid completely penetrates the other two components. The mixture is ejected from the apparatus through a pressure release valve and a discharge nozzle. The patented process provides neither sufficient pressure drop nor temperature to cause total expansion of the foam. At column 5, lines 54-58, it is disclosed that approximately 20 percent post-expansion occurs.